It is known to provide pyrolytic coating-removal apparatus in which metallic and ceramic articles may be delacquered by subjecting them to a pyrolytic treatment in which the coating material partially evaporates, partially decomposes thermally, and partially burns so that the metal and ceramic objects can be freed after such coating.
Generally, the apparatus can include a burner which opens into a combustion chamber which generates the heat required for the delacquering process, an afterburner chamber and a fluidized bed retort which can be composed of refractory steel.
Via appropriate duct work or passages, incompletely combusted gases, i.e. gases containing pyrolytic decomposition products which remain combustible or contain combustible components, such as retort gases, are collected from the fluidized bed and burned in the afterburner chamber. The fluidized-bed retort can be a nozzle chamber with a floor formed by nozzles through which the carrier or fluidizing gas can be fed.
The carrier gas is usually air or oxygen-enriched air and is forced through the nozzle chamber or plenum communicating with nozzles by a blower.
The retort gas, i.e. the distillation residue-containing gas, is formed during the pyrolytic removal of the lacquer from the articles in the fluidized-bed retort.
A lacquer sludge which may arise from the coating apparatus can be introduced into the fluidized bed of the retort to add combustibles to the latter. Consequently, the distillation gases will contain complete combustion products, partial combustion products and other combustibles, and even incompletely burned or fully unburned components. For convenience in description, we will refer to an "exhaus gas" which is the afterburned gas which is discharged from the apparatus.
The afterburning of the distillation gases is carried out so that this step will remove all detrimental or noxious components which may be environmentally hazardous by burning so that the exhaust gas which is ultimately discharged will be free from noxious or toxic components.
The after-combustion also has the advantage that it allows recovery of the energy represented by combustible components in the distillation gases, energy which can be retrieved and utilized in the fluidized-bed retort for the delacquering process.
The objects which are to be treated can be suspended in the fluidized bed retort, for example in baskets, and the fluidized bed itself can be constituted of granules or particles of a material which is not significantly depletable, for example, quartz grains.
In general, one can provide adjacent the fluidized-bed retort a depositing grate upon which the hot products, after delacquering, can be placed.
In the conventional apparatus of this type, the distillation gas is fed to the afterburner chamber and is there burned with the detrimental components being decomposed and the energy of this combustion being utilized. The delacquering is carried out discontinuously in that the objects to be delacquered are introduced basket by basket into the fluidized-bed retort. The distillation gas which is produced also is therefore generated in surges or discontinuously or, in the case when a carrier gas is passed continuously through the system, the combustible components will appear in surges in this gas. The exhaust gases are therefore formed in an afterburner chamber which is subjected to high temperature variations or fluctuations ranging from 450.degree. C. to 110.degree. C., for example, and it is for this reason that the fluidized-bed retort cannot be heated directly by the combustion product of the afterburner chamber. The sensible heat of the combustion products of the afterburner, therefore, must be recovered in other ways.
One of the principal problems of the earlier systems has been the different units for combustion and afterburners which were used as the ducting systems thus required. High flow resistance, problems in flow control and problems with uniform temperature distribution, particularly when the retort had a rectangular cross section, were experienced.